Cathode ray tubes are increasingly being used as visual display terminals (VDT's) which are scanned at close range by the human eye. The higher anode voltages which are required for larger size tubes has resulted in the possibility of increased electric shocks to the human body rising from an electrostatic charge induced by turning the CRT switch on and off. Additionally, static electricity charges cause dust to adhere to the CRT glass surface, resulting in the reduction of brightness and contrast of the CRT screen. Also, it is desirable to minimize the glare that is reflected from the glass surface of the CRT so as to enable the user to more easily read the graphics and other display characters that are shown on the screen.
It is known to reduce the glare and static charge on CRT face panels by applying a double layer of ultra fine tin oxide particles onto the surface of the face panel. The tin oxide particles, having a diameter of about 50 nm, are suspended in a solution of ethyl silicate and ethanol. The suspension of tin oxide particles is coated by a spinner onto the exterior surface of the base plate of the CRT to produce a transparent, electro-conductive layer. The coated surface is heated after the application of the tin oxide layer for about thirty minutes at a temperature in the range of 100.degree. C. to 200.degree. C. Thereafter, a second layer of ultra-fine 50 nm diameter tin oxide particles suspended in a solution of ethyl silicate and ethanol is coated onto the first layer by a spinner to produce a non-glare layer. The CRT tube with the two layers of tin oxide particles are again heated for about thirty minutes at temperatures in the range of 100.degree. C. to 200.degree. C.
Untreated CRT's have a surface resistivity of about 10.sup.13 ohms to about 10.sup.14 ohms. After treatment by the above-described two-step tin oxide process, the CRT surface resistivity is reduced to about 10.sup.7 ohms. Untreated CRT's have a reflectivity of about 4.9% (at 550 nm). CRT's which have been treated in accordance with the above-described two-step tin oxide coating process have a reflectivity of about 0.5% (at 550 nm).
U.S. Pat. No. 4,563,612 to Deal, et al. describes a cathode ray tube having an antistatic, glare-reducing coating. The coating has a rough surface which is composed essentially of a silicate material and an inorganic metallic compound. The coating is applied by spraying a solution of a water soluble salt of one or more of a metal selected from platinum, tin, palladium and gold in a lithium stabilized silica sol onto the surface of the cathode ray tube. A solution of lithium, sodium and potassium silicate or an organic silicate, such as tetraethyl orthosilicate may be substituted for the lithium stabilized silica sol.
U.S. Pat. No. 3,689,312 to Long, et al. is directed to a method for producing a glare-reducing coating on the surface of a cathode ray tube. The method includes the steps of preparing a coating formulation consisting of a solution of a siliceous polymer and an organic polymer in a volatile organic liquid vehicle for the polymers. The solution is then sprayed onto the surface of a cathode ray tube to coat the surface. The cathode ray tube is then baked at a temperature of 100.degree. C. to 200.degree. C. to cure the coating.
A cathode ray tube having an antistatic film is disclosed in U.S. Pat. No. 4,785,227 to Matsuda, et al. The antistatic film is applied by dipping the cathode ray tube into a mixture of tetraethyl silicate, propanol and butanol containing a colloidal solution of metal particles.
U.S. Pat. No. 4,945,282 to Kawamura is directed to improving the antistatic and antiglare properties of cathode ray tubes. The Kawamura, et al. patent utilizes particles of a metal oxide or a hygroscopic metal salt, in various formats, to provide improvements in antistatic and antiglare properties. See Column 5, lines 2-5 and claim 1 of the Kawamura, et al. patent for a specific teaching of the use of metal oxide and hygroscopic metal salt particles. The present invention utilizes a solution of a metallic compound to provide improved antistatic and antiglare properties and forms a metal oxide in situ on the surface of the cathode ray tube.
As set forth in the Kawamura, et al. patent, it is indicated that the transparent conductive film, particularly in the second embodiment, may be a thin SiO.sub.2 film into which a transparent conductive metal oxide particle and/or hygroscopic metal salt particles have been incorporated to impart conductivity. The hygroscopic metal salt particle contained in the thin SiO.sub.2 film is preferably a metal salt of group 2 metals, represented by magnesium, and salts of group 3 metals, represented by aluminum. There is no teaching or suggestion in the Kawamura, et al. patent that such salts may be converted to the oxide during a curing step. Instead, the Kawamura, et al. patent indicates that the hygroscopic metal salts absorb moisture from the atmosphere to lower the electric resistance of the panel surface. If an oxide of aluminum or magnesium were to be formed, the metal salts would lose their ability to absorb moisture from the atmosphere and to lower the electric resistance of the panel surface in accordance with the theory of the Kawamura, et al. patent. Indeed, forming an oxide of aluminum or magnesium would be highly unlikely considering the conditions under which the siloxane film is cured in accordance with the Kawamura, et al. patent, i.e., at a temperature of from about 50 to 200.degree. C.
While the two-step (two coating and curing steps) tin oxide coating process described above provides a CRT with improved antiglare and antistatic properties, it would be desirable for reasons of economy and reduced handling, to provide a one-step (one coating and curing step) process wherein antiglare and antistatic properties are similarly improved.